Emergency lighting, sometimes referred to as egress lighting, is lighting that is activated in the event of power loss. One purpose of emergency lighting is to allow occupants of a building to safely exit the building in the event of a power outage or other emergency. Emergency lighting is mandated for use in commercial buildings by many electrical codes. Such codes generally specify the amount of light that must be provided in the event of power loss and the duration of time for which such light must be provided. For example, U.S. building codes require emergency lighting to provide one footcandle of light for a minimum of 90 minutes along the path of egress during a power outage.
Emergency lighting fixtures typically have a test button which temporarily overrides the unit and causes it to switch on the lights and operate from battery power even if the main AC line power is still on. Typically, the test button must be manually operated by a technician, and may be held down for the duration of the test.
In buildings, emergency lighting is commonly provided by battery-powered emergency light fixtures that are installed in a building along with the luminaires that provide light in non-emergency situations. In some systems, emergency lights are powered by a central bank of batteries. Building codes generally required the wiring from the central power source to emergency luminaires to be isolated from other electrical wiring.
For fluorescent lighting fixtures, emergency operation may be controlled by an emergency ballast that includes a backup battery. A typical fluorescent emergency lighting fixture is illustrated in FIG. 1. The lighting fixture 10 includes an emergency ballast 12 that includes a backup battery 24. A two-lamp instant start ballast 14 is connected to two fluorescent lamps 16A, 16B. A test switch 18 permits manual activation of the emergency ballast 12.
As is known in the art, fluorescent lamp ballasts stabilize the current through fluorescent lamps, which have a negative resistance characteristic. The ballast provides a positive resistance or reactance that limits the current through the fluorescent lamp to an appropriate level. An instant start ballast, such as the ballast 14, starts the lamps 16A, 16B without heating the cathodes of the lamps by generating a high initial voltage (around 600 V).
A fluorescent emergency lighting system can also be configured so that the emergency ballast 12 serves the function of both providing regular illumination and emergency lighting without the need for a separate lamp ballast.